1. Field of the Invention
The present invention pertains to methods of roughening a ceramic surface in order to promote adherence of a material applied over the ceramic. The invention also pertains to components for use in semiconductor processing equipment which include roughened ceramic surfaces.
2. Brief Description of the Background Art
In semiconductor device manufacturing, physical vapor deposition (PVD) is a process which is frequently used to deposit a layer of material onto a substrate. FIG. 1 shows a cross-sectional schematic of a PVD processing chamber 100. During a PVD process, a plasma (such as an argon plasma) is used to sputter material (such as copper or tantalum) from a target 102 onto the surface of a semiconductor substrate 104 (typically a silicon wafer), which sits atop an electrostatic chuck 106. A deposition ring 108 is positioned over the exposed upper surface of the chuck 106 which extends beyond the outer edge of semiconductor substrate 104, in order to protect the chuck from depositing materials. Deposition ring 108 is typically made of a ceramic material such as aluminum oxide, so that the linear thermal expansion of the deposition ring 108 will be the same as the aluminum oxide surface of the electrostatic chuck 106. A cover ring 110 encircles the outer edge of deposition ring 108. The cover ring 110 is typically made of a metal, such as titanium.
During copper metallization processes, a layer of tantalum is frequently deposited onto the substrate 104 as a wetting layer to facilitate subsequent copper deposition. During tantalum deposition (and also during chamber warm-up operations, when a tantalum target is in the chamber), tantalum is sputtered onto the deposition ring 108, as well as the substrate 104. The ceramic surface of the deposition ring 108 is roughened so that the depositing tantalum will adhere to the surface of the deposition ring 108 and will not flake off and contaminate the chamber. Roughening of the ceramic surface of the deposition ring is typically performed by grit blasting using silicon carbide particles.
At some point, the tantalum build-up must be removed from the deposition ring 108, before the amount of deposition becomes so great that the tantalum bridges across to surfaces adjacent the deposition ring 108 and creates an electrical pathway between the metal cover ring 110 and the semiconductor substrate 102. However, tantalum is highly resistant to chemical etchants and is not easily removed by conventional means.
Referring to FIG. 2A, one approach to provide for tantalum removal involves coating the roughened surface 203 of the ceramic deposition ring 202 with a sacrificial layer of aluminum 204. FIG. 2B shows the surface 203 of the ceramic deposition ring 202 as a layer 206 of tantalum starts to build up over sacrificial aluminum layer 204. The aluminum layer 204 can be easily dissolved away (e.g., by dipping in an acid bath), taking the overlying deposited tantalum 206 with it (not shown). However, as the tantalum layer 206 builds up during semiconductor processing operations, it pulls on the underlying sacrificial aluminum layer 204, causing the aluminum layer 204 to separate from the surface 203 of ceramic deposition ring 202, as shown in FIG. 2C. This usually results in flaking off of the dual layer of tantalum 206 and aluminum 204.
Although the nature of this failure is currently not well understood, initial observations indicate that the failure occurs close to the interface 203 between the sacrificial aluminum layer 204 and the ceramic deposition ring 202, and not deep within the aluminum layer. It is therefore believed that surface properties of the ceramic are a major contributing factor in the observed failures.
The adherence of the aluminum layer 204 to the underlying ceramic surface 203 is principally determined by the tensile strength of the ceramic matrix (which affects cohesive strength) and the surface morphology of the ceramic (which affects surface adherence). Roughening of the ceramic surface 203 by diamond tool grinding is known to create microcracks 205 in the first few microns of the ceramic surface, thereby reducing the tensile strength of the ceramic matrix and increasing the brittleness of the ceramic, subjecting the ceramic material to cohesive failure when the overlying sacrificial aluminum layer 204 places stress on the surface 203 of ceramic deposition ring 202. It is expected that silicon carbide grit blasting has a similar effect on the ceramic as diamond tool grinding, as grit particles impact and may even become embedded in the ceramic surface. Therefore, stresses created within the ceramic deposition ring 202 due to tensile forces applied by the pulling of the tantalum layer 206 and aluminum layer 204 as they separate from the ceramic 202 are expected to further increase the depth and the extent of the microcracking 205, as shown in FIG. 2C.
There is a need for a method of roughening a ceramic surface which promotes adherence of the sacrificial aluminum layer 204 to the surface 203 of the ceramic deposition ring 202, while minimizing types of damage which promote initiation of or increase in the cracking of the ceramic surface. It is well established that sharp reentrant corners in the surface or outer layers of a brittle material can be sites of crack initiation under stress conditions.